The present invention relates to a solvent-based, dual cure coating incorporating thermally curable blocked polyisocyanate resins, which also contain unsaturated linkages.
The hardening of coating compositions containing (meth)acryloyl groups by UV-initiated polymerization which proceeds by a free radical mechanism is a known and industrially established procedure. It is one of the most rapid methods of hardening in coating technology. However, UV-hardening lacquers have certain disadvantages, such as their high volume shrinkage during hardening, which can result in adhesion problems. The high crosslink density which is inherent in the system results in brittleness, lack of flexibility and a lack of impact strength. Also, the hardening of coating compositions containing UV-hardening (meth)acryloyl groups is dependent on the availability of a sufficient dosage of radiation of the appropriate wavelength. Poorly exposed regions suffer from reduced hardening, which results in appreciable losses in surface resistance, and unexposed regions do not harden at all.
Also well known and widely used in industrial practice is the preparation of coatings from aqueous compositions derived from polyisocyanates and compound with isocyanate-reactive groups. Due to the wide choice of reactants, the technical properties of these coatings, such as adhesion, flexibility, impact strength and resistance to weathering, can be varied over a wide range. However, compared with radiation-hardening systems, the hardening of aqueous two-component polyurethane compositions requires a long time, even at elevated temperatures. Another difference is that UV light is not necessary for curing.
Combinations of both types of hardening, i.e., so-called dual cure systems hardening by UV radiation and hardening by polyurethane formation, are also known for solvent-based lacquers. For example, U.S. Pat. No. 4,342,793 describes hardenable resin compositions which contain a radiation-hardenable reactive thinner (low molecular weight esters of acrylic acid), a saturated polyol and a polyisocyanate. Hardening is carried out by irradiation in order to polymerize the reactive thinner, which is followed by thermal hardening to form a polyurethane from the polyol and the polyisocyanate. One-component dual cure coatings are also known (see U.S. Pat. No. 5,234,970).
Dual cure systems have varying processing (e.g. processing speed, pot life and cure conditions) and end use demands (e.g., thermal characteristics, weathering and hydrolytic stability). Accordingly the intended application and the environment related to that application have typically dictated the particular formulation of the coating.
U.S. Pat. No. 5,350,823 discloses the preparation of a self-crosslinkable copolymer of a) monomer prepared by first reacting isophorone diisocyanate (IPDI) with 2-hydroxyethyl acrylate and then with butanone oxime, and b) an usaturated compound. Crosslinking of the copolymer occurs by a single mechanism only.
U.S. Pat. No. 4,485,226 describes a UV-curable composition which is a combination of a) a blocked isocyanate prepared by reacting a portion of the isocyanate groups of an aliphatic isocyanate with butanone oxime and then reacting the rest of the isocyanate groups with hydroxyethylacrylate and b) an isocyanate reactive material (see column 5, lines 19–31). As described therein, after polyurethane formation, the systems are converted to the cross-linked state by high-energy radiation.
U.S. Pat. No. 5,908,907 describes aqueous emulsions from blocked isocyanates wherein a portion of the isocyanate groups of an HDI trimer are blocked with methyl ethyl ketoxime and then the rest of the isocyanate groups are reacted with hydroxyethylacrylate.
U.S. Pat. No. 4,008,247 discloses reacting a portion of the isocyanate groups of an aromatic isocyanate with an hydroxy functional unsaturated monomer and the rest of the isocyanate groups with a blocking agent (including dimethylpyrazole). Ethylenically unsaturated blocked (cyclo)aliphatic diisocyanates are not described.
There remains a need for dual-cure coatings formulations, producing desirable coatings properties, in which blocked (cyclo)aliphatic polyisocyanates are combined in the same molecule with UV cure capabilities.